Feeding My Ball Python Rats?

My vet said it is now time for Sammy, a ball python, to move up to eating rats. Last night I offered him his first one. He refused it. I must add in I know that these snakes can be picky, but Sammy has always, always been a good eater. Did he refuse because he knows it is not the typical mouse he has eaten in the past? Does he know the difference? Or perhaps was he just not wanting food.